1. FIELD OF THE INVENTION
The instant invention relates to a process for the manufacture of powders for superconductive ceramic materials.
In particular, the invention relates to the manufacture of powders of superconductive ceramic oxides of the type Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x (YBCO), x being between 0 and 1, bounds included, by means of the pyrolysis of a solution containing the ions of the metals forming the same oxides.
2. THE PRIOR ART
It is known that ceramic materials, like the ones belonging to the class of the complex oxides such as YBCO, can show superconductivity at a temperature higher than that of liquid nitrogen.
Such a feature is considerably interesting, from a practical/applicatory point of view, but the interest is tempered and restrained either by the poor availability of oxides showing suitable features or by the high manufacturing cost of such oxides.
Particularly important, and obtainable with difficulty, is the combination of homogeneity and fine granulometry of the starting powders of such oxides, which have to subsequently undergo compacting (hot-pressing) and syntering.
In the case of YBCO, the most commonly used methods, exploiting the solid-state reaction of a mixture of copper oxide, yttrium oxide and barium carbonate, exhibit a drawback because it is very unlikely that a single-phase compound can be obtained by means of only one thermal treatment (at high temperature) and high amounts of barium carbonate thus remain unreacted. That's why further onerous annealing treatments are required, which treatments cause an undesired agglomeration of the powders. The agglomerated powders must then undergo a grinding. In particular, in the case of YBCO, the grinding cannot be wet-ground in water, but only in the presence of an organic solvent, which obviously involves further costs and problems.
Therefore, the preliminary techniques described hereinabove are considered particularly unfavourable.
The chemical methods for the manufacture of the powders are for instance the sol-gel method, coprecipitation, and pyrolysis. Such methods are more reliable and give rise to powders having a homogeneous composition and a fine granulometry, but they are complex, critical and expensive in the peculiar case of YBCO. In particular, the pyrolysis method for the manufacture of YBCO, as described for instance on Nov. 8, 1988 by F. Celani et al., in the report 88/62(P) of the Ist. Naz. di Fisica Nucleare (Lab.Naz.Frascati) entitled "PYROLYTIC CITRATE SYNTHESIS AND OZONE ANNEALING. TWO KEY STEPS TOWARD THE OPTIMIZATION OF SYNTERED YBCO", gives rise to such a low yield that industrialization of this process is not justified.
The pyrolysis method described hereinabove is based on the combustion of a solution of citrates of the metals building up the YBCO, said solution being made flammable by the presence of ammonium nitrate. Such a method imparts to the dry pyrolysis residue the same microscopic homogeneity that is typical of the starting solution, provided that the solution is clear without showing any suspended body. The reaching of such a condition is, however, critical and can be reproduced only with difficulty.
Moreover, the thus obtained powders are not single-phase and must undergo long (tens of hours), critical (because of powder agglomeration) thermal post-treatments, in order to attain single-phase features.
The Applicant found that by working in a specific way and by a strict control of the operative conditions, the drawbacks described hereinabove can be overcome, thus obtaining a highly satisfactory reproducibility and a constant quality of the oxides, in terms of homogeneity of the composition and of granulometry.